Osteohistology of a Triassic dinosaur population reveals highly variable growth trajectories typified early dinosaur ontogeny

Intraspecific variation in growth trajectories provides a fundamental source of variation upon which natural selection acts. Recent work hints that early dinosaurs possessed elevated levels of such variation compared to other archosaurs, but comprehensive data uniting body size, bone histology, and morphological variation from a stratigraphically constrained early dinosaur population are needed to test this hypothesis. The Triassic theropod Coelophysis bauri, known from a bonebed preserving a single population of coeval individuals, provides an exceptional system to assess whether highly variable growth patterns were present near the origin of Dinosauria. Twenty-four histologically sampled individuals were less than a year to at least four years old and confirm the right-skewed age distribution of the Coelophysis assemblage. Poor correlations among size, age, and morphological maturity strongly support the presence of unique, highly variable growth trajectories in early dinosaurs relative to coeval archosaurs and their living kin.


Additional Histology Methods
Cleveland Museum of Natural History (CMNH) specimens: Because sampling of a full crosssection of long bones was not possible for CMNH Coelophysis bauri specimens, we instead sampled fragments taken from the midshaft of long bones. For the fibula of CMNH 10971 #1, we sampled a posterior portion of the left fibula, and additionally sampled fragments from both the medial and lateral sides of the cortex of the right femur from this individual. We also sampled a fragment from the anterior half of the right fibula of CMNH 10971 #5.
We embedded the fragments in Castolite-AC resin, placing the wet resin under vacuum for ~2 minutes to ensure the resin fully embedded the samples. After the resin set, we used a Buehler Isomet 1000 diamond-bladed saw to cut 1.5-mm thick transverse wafers of the embedded bone. Following this, we polished one side of each wafer using a Buehler grinding/polishing wheel and glued the polished side to a plexiglass microscope slide using cyanoacrylate glue. We ground the glued wafer down to an acceptable thickness to see histological detail using the grinding/polishing wheel and polished when this thickness was reached. We viewed slides with an Olympus BX51 petrographic microscope under planepolarized light, cross-polarized light, and cross-polarized light under a 530 nm gypsum wave plate. Histological images were captured with a Luminera Infinity 1 microscope camera, and whole-slide images were stitched together using Adobe Photoshop version 21.2.1.
Royal Ontario Museum (ROM) specimen: We embedded the broken distal end of ROM 72668 in Castolite AP resin and cut a wafer using a Buehler Isomet 1000 diamond-bladed saw. We frosted and glued together both the wafer and a plexiglass slide with cyanoacrylate glue. The wafer was then trimmed further with the Buehler Isomet 1000 saw. We ground the resulting thick section to an acceptable thickness to observe histological detail using a Hilquist 1010 thin sectioning machine. We polished the thin section by hand using progressively finer (600 to 1000) silicon carbide grits and water on a plexiglass plate. We viewed the specimen under plane-polarized and cross-polarized light and captured digital photomicrographs using a Nikon Az100 petrographic microscope. The photomicrographs were stitched together using Nikon Elements imaging software.

Growth Rate Calculation
To determine if Coelophysis bauri tibiae lacking growth marks could have grown to their preserved circumference within a year, we constructed a simple geometric model to test the assumption that they represent yearling or sub-yearling individuals. The preserved circumferences of the sampled C. bauri tibiae lacking growth marks are all under 31 mm (excluding the histologically abnormal ROM 72668) (Supplementary Table 1). Modeling a circular tibial midshaft with a 31 mm circumference (radius ~ 4.9 mm) gives a growth rate of 12.8 μm/day, assuming that the entire radius was deposited within a 381-day Triassic year 1 . Accounting for a possible 90-day growth hiatus [e.g., ref. 2 ] gives a growth rate of 16.8 μm/day. Assuming a one-half year growth hiatus of 190.5 days gives a growth rate of 25.7 μm/day. Of course, calculating the radius in this way does not factor in the size of the medullary cavity, so all estimates provided here are certainly overestimates of growth rate.
Alternatively, we can use the measured first growth zones to model the growth rate, though these may preserve less than the amount of bone deposited in a year, due to medullary cavity expansion. None of the measured first growth zones exceed approximately 2,300 μm ( Fig.  6). Using this number for ease of calculation gives hypothetical growth rates of 6.0 (full year), 7.9 (90-day hiatus), and 12.1 μm/day (190.5-day hiatus).
All calculated rates lie within the range of fibrolamellar and lamellar/parallel-fibered bone in extant mallard ducks (Anas platyrhynchos) 3 , and although the high growth rates of extant birds make them difficult to directly compare with Coelophysis growth rates they provide an upper bound of what is reasonable for given tissue types. Previous estimates of the growth rate of another coelophysid specimen [University of California Museum of Paleontology (UCMP) 129618] found a range of 15-17.5 4 or 5-20 μm/day 5 , very similar to the rates we calculated. The rates are similar to the maximum calculated for the later ontogeny of tibiae of the large theropod Tyrannosaurus rex 6 and slower than those of the first three years of tibial growth of the large ornithopod Maiasaura peeblesorum 2 . This is expected for a smaller dinosaur that grew more slowly early in life 5 . Therefore, the Coelophysis appositional growth rates presented here do not seem unrealistically high for a dinosaur, so it is plausible that individuals with zero growth marks died sometime during their first year of life. This supports our assumption that a substantial number of growth marks are not missing due to medullary cavity expansion or other processes in the zero-growth mark specimens.